Backport of ad720c4 for oC 4.x

Fixes #145

lib/util.php

@@ -437,6 +437,11 @@ class OC_Util {
 
 		// creating a test file
                 $testfile = OC_Config::getValue( "datadirectory", OC::$SERVERROOT."/data" ).'/'.$filename;
+
+                 if(file_exists($testfile)){// already running this test, possible recursive call
+                 	return false;
+                 }
+                 
                 $fp = @fopen($testfile, 'w');
                 @fwrite($fp, $testcontent);
                 @fclose($fp);